PNAS highlights for the week of April 4 - 8

Ancient Maya Set Up Remote Salt FactoriesThis tip is also available in Spanish

Ancient Maya entrepreneurs set up extensive workshops beyond royal control on the Caribbean coast, where they produced salt for river transport to inland cities, archaeological findings suggest.

From 600-900 A.D., the need for salt was great in large urban Maya populations in the interior of the Yucatan peninsula, in what is now Mexico, Guatemala, and Belize. Heather McKillop reports the results of an underwater archaeological survey of the mangrove peat bog of Punta Ycacos Lagoon on the south coast of Belize. Before her search, a handful of salt workshops had been found in the lagoon and farther north along the coast, but the extent and details of regional salt-making were unclear.

The author's team of snorkeling researchers found evidence of 41 additional salt works, with remains of wooden buildings at more than half of them. Ceramic pottery remains suggest that Maya workers boiled seawater to retrieve salt, and recovery of a wood paddle ties the salt production to river delivery by canoes.

The workshops were far from royal Maya leaders in the cities, the author says, indicating an economy more complex than previously thought.

Reducing the "Toll" of Nerve Pain

According to a newly published study, the Toll-like receptor 4 (TLR4) plays a critical role in inducing neuropathic pain--a debilitating condition in which nerves generate pain by themselves, without a painful stimulus.

TLR4 is expressed exclusively by microglia, immune cells of the central nervous system (CNS) that become activated soon after an injury. Following nerve injury in animals, TLR4 has been implicated in behavioral hypersensitivity--a model of neuropathic pain whereby the CNS overreacts to sensory input.

To determine if TLR4 contributes to neuropathic pain, Flobert Tanga and colleagues severed a nerve in the lower back of mice lacking functional TLR4. To assess hypersensitivity, the researchers observed how the mice reacted when their hindpaws were touched or exposed to heat. TLR4-deficient mice were less sensitive than normal mice and exhibited decreased expression of activated microglia.

These results demonstrate that TLR4 contributes to the initiation of the CNS immune response, leading to behavioral hypersensitivity. Further understanding of TLR4's ability to activate hypersensitivity may provide an opportunity to regulate microglial activation and alleviate chronic pain due to nerve damage.

Air Pollution Impacts Wheat Disease

Atmospheric pollution correlates with the prevalence of wheat diseases caused by two fungal pathogens, researchers report.

Some agricultural diseases can suddenly appear or disappear, yet disease dynamics can rarely be analyzed due to lack of long-term data. To elucidate factors controlling wheat disease patterns in the United Kingdom, Bart Fraaije and colleagues measured the abundance of DNA for two fungal pathogens (Phaeosphaeria nodorum and Mycosphaerella graminicola) in wheat samples archived from a long-term experiment that began in 1843.

The scientists determined that the relative amounts of pathogen DNA correlated with national wheat disease trends. M. graminicola, the most abundant pathogen since the 1980s, was also prominent in the mid-1800s. In contrast, P. nodorum DNA was more common than M. graminicola for much of the 1900s, with a peak in about 1970. Unexpectedly, long-term changes in the ratio of the pathogens were strongly correlated with changes in air pollution, as measured by sulfur dioxide (SO2) emissions. When emissions were high, P. nodorum was abundant, while M. graminicola was scarce--indicating that SO2 differentially affected pathogen growth or impaired plant disease resistance.

The researchers suggest that similar research could assess how the changing environment impacts biodiversity and possibly predict disease outbreaks.

Characterizing Skin Cancer by Microarrays

Microarray technology can be used to identify each stage of skin cancer tumor (melanoma) progression, according to a newly published report.

Melanomas grow in two phases: radial and vertical. Vertical-phase tumors are more likely to metastasize (spread cancer to other parts of the body) than radial melanomas. Christopher Haqq and colleagues dissected a large primary melanoma into radial and vertical portions, and through microarray techniques found that, in the transition from radial to vertical growth, the melanomas lost the expression of certain molecules, resulting in increased risk of metastasis. However, some metastatic tumors retained a gene expression pattern characteristic of the radial growth phase, which implies that both radial and vertical melanomas can metastasize. The authors identified over 2,000 genes that distinguished the stages of melanoma progression.

The authors say that their results will provide a basis for developing new molecular diagnostic techniques and, eventually, targeted therapies for patients with skin cancer.

Emotion and Cognition Set Up by Serotonin

Cognition and emotion may depend on tightly controlled serotonin levels in the developing brain, according to a newly published study.

Altered levels of serotonin, the brain's "feel-good" chemical transmitter, underlie disorders ranging from addiction to schizophrenia. Louis Sokoloff and colleagues examined the brain pathway from the whiskers to sensory processing in mice lacking a protein that transports serotonin. Lacking this transporter is hypothesized to lead to elevated serotonin levels in the brain.

The researchers stimulated individual whiskers and found that the mice displayed decreased brain activity. These deficits were prevented by administering inhibitors of serotonin synthesis shortly after birth, suggesting that the deficits are due to abnormally high levels of serotonin in the brain during development.

The results illustrate the importance of serotonin in the developing brain and imply that disturbances in proper serotonin levels may lead to long-lasting functional and behavioral abnormalities.